1. Field of the Invention
The present invention relates to hydrophobically modified polymers and more particularly it relates to hydrophobically modified water soluble comb copolymers suitable for thickening water-borne systems.
2. Description of the Prior Art
Water soluble polymers (also commonly referred to as "thickeners" or "rheology modifiers") are widely used in many industrial water-borne systems as additives to modify their flow behavior. One such highly filled aqueous system is the latex paint, which is composed of a dispersion of a polymeric latex, pigment and clays and other additives in water.
Typically, a small amount (0.1-5 wt %) of water soluble polymer is added to latex paints to achieve the following performance characteristics during manufacturing, storage, and applications:
a) Ease of formulation and ability to manufacture at a fast rate, PA0 b) Prevention of settling of the suspended particles (latex, pigment, etc.) during storage, PA0 c) Good film build during applications to achieve efficient hiding without excessive brush or roller drag, PA0 d) Good roller spatter resistance, PA0 c) No excessive sagging after application on a vertical surface, and PA0 e) Good flow and leveling for the formation of a smooth and continuous film with good appearance. PA0 (1) copolymerizing water soluble polymer bearing alpha, omega-active hydrogen, PA0 (2) hydrophobic compounds having alpha, omega-active hydrogen atoms or their alkoxylated derivatives, and PA0 (3) dihalogeno compounds or their derivatives PA0 (1) water soluble (hydrophilic) polymer bearing alpha, omega-active hydrogen atoms such as polyalkylene oxide, copolymer of polyalkylene oxide, poly(acrylic acid), poly(acrylic acid-co-methacrylic acid), poly(acrylamide), poly(dialkyldiallylammonium salts), polyamidopolyamide, poly(ethyleneimine), poly(methyl vinyl ether-co-maleic anhydride), polyvinylpyrrolidone, poly(2-ethyloxazoline) and polypeptides. PA0 (2) hydrophobic compounds having alpha, omega-active hydrogen atoms or their alkoxylated derivatives, such as alkylene oxide modified fatty amines, and ethoxylated organosilicon or organophosphorous compounds having hydrophobes compounds groups attached thereto, and PA0 (3) dihalogeno compounds or their derivatives PA0 H--B(R.sub.1)--H=a compound comprising a chemical entity "B", which can be an atom or a group of atoms, having a valence of at least three and having attached to "B" alpha, omega-active hydrogen atoms and a pendant hydrophobe group "R.sub.1 " or a mixture thereof; PA0 R.sub.1 =hydrocarbyl, alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, cycloaliphatic, perfluoroalkyl, carbosilyl, fullerenyl, polycyclic and complex dendritic groups; PA0 X--D(R.sup..vertline..sub.1)--X=a dihalogeno compound wherein D is a hydrocarbon moiety, preferably having from 1 to 20 carbon atoms, or organometallic radical, such as organosilyl radical or organophosphoryl radical or a mixture thereof; PA0 R.sup..vertline..sub.1 =H or a hydrophobe radical or organometallic radical; PA0 X=halogen, such as Br, Cl, etc.; PA0 R.sub.2 --Y=hydrophobic compound capable of reacting with terminal active hydrogens of comb copolymer (I) through its functional group Y; PA0 R.sub.2 =hydrocarbyl, alkyl, alkenyl, aryl, arylalkyl, arylalkenyl, cycloaliphatic, perfluoroalkyl, carbosilyl, fullerenyl, polycyclic and complex dendritic groups; PA0 m, n and p represent the number of units of --A--, --B(R.sub.1)-- and --D(R.sup..vertline..sub.1)-- in comb copolymer (I) and comb copolymer (II), preferably m=1-50; n=1-20; p=1-10.
Various water soluble polymers currently being used in latex paints are: i) natural polysaccharides, ii) chemically modified polysaccharides, and iii) synthetic polymers. Examples of polysaccharide based thickeners include xanthan gum, locust bean gum, and cellulosics such as hydroxyethylcellulose, methylcellulose, hydroxypropylmethylcellulose and ethylhydroxyethylcellulose. Synthetic thickeners are polyacrylates, polyalkylene oxides and polyacrylamides which could be homopolymers or copolymers. Their hydrolytic stability and performance properties are, however, pH dependent.
The above mentioned natural and synthetic thickeners provide different degrees of thickening efficiency and application properties. However, they invariably fail to provide key performance properties in gloss paints. These include good film build, flow and leveling, and gloss which are generally offered by solvent based alkyd paints. Another drawback of these thickeners is that they have poor compatibility with the various paint ingredients.
To eliminate some of the performance deficiencies of conventional thickeners, a new class of thickeners, commonly referred to as "associative thickeners", has recently been designed and commercialized (See E. J. Schaller and P. R. Sperry, in "Handbook of Coatings Additives", Ed. L. J. Calbo, Vol. 2, p. 105, 1992; Marcel Dekker, Inc., New York). These thickeners are hydrophobically modified water soluble polymers. They undergo intermolecular association in aqueous solution and thereby exhibit enhanced solution viscosity. They can also adsorb onto the dispersed phase particles of an aqueous dispersion and thereby form a three dimensional network. Since they provide improved paint properties not offered by conventional thickeners, they have gained commercial importance.
Polysaccharide based associative thickeners are made by chemically grafting a small amount of a hydrophobic group (C.sub.10 -C.sub.24 alkyl) onto the polysaccharide backbone and they are disclosed in U.S. Pat. No. 4,228,277; 4,243,802 and EP 281,360.
Among commercial nonionic synthetic associative thickeners, hydrophobically modified ethylene oxide urethane (HEUR) block copolymers constitute an important class. They are disclosed in U.S. Pat. Nos. 4,079,028, 4,155,892, and 5,281,654. These are low molecular weight polyurethanes made by condensing relatively low molecular weight polyethylene glycol (molecular weight .about.10,000) with hydrophobic diisocyanates and end-capping with hydrophobic alcohols or amines. They are characterized by having three or more hydrophobes--two of which are terminal and the remainder are internal. The hydrophobic groups are connected to the hydrophilic polyethylene oxide blocks through urethane linkages.
The preparation of water dispersible HEURs is disclosed in U.S. Pat. Nos. 4,499,233 and 5,023,309. These HEURs are claimed to provide superior viscosifying properties and improved leveling in aqueous systems.
Processes for the production of HEURs with pendant hydrophobes in bunches are described in U.S. Pat. Nos. 4,426,485 and 4,496,708. These HEURs are believed to provide enhanced thickening to aqueous systems through micelle-like association.
HEURs with branched structure and terminal hydrophobes are disclosed in U. S. Pat. No. 4,327,008. They are made by reacting polyalkylene oxides with a polyfunctional material, a diisocyanate and water and end-capping the resulting product with a hydrophobic monofunctional active hydrogen-containing compound or a monoisocyanate.
Silicon-containing HEURs having particular utility in paints and coatings are disclosed in European Patent Application No. 0498,442 A1. These are reaction products of isocyanate functional material, a polyether polyol, a monofunctional active hydrogen containing hydrophobic compound, a silane-functional material, and water.
One of the drawbacks of HEUR thickeners is that they are hydrolytically unstable under strongly acidic or alkaline conditions, particularly at elevated temperatures. Hence, they cannot be processed or used in such situations.
From applications and economic point of view, the major drawbacks of HEUR thickeners are their high cost, difficulty in handling, and tendency to destabilize the paint (separation of dispersed particles from the aqueous phase) (G. D. Shay and A. F. Rich, J. of Coatings Technology, Vol. 58, No. 7, p.43, 1986).
Random copolymers of ethylene oxide and long chain alkyl epoxides are disclosed in U. S. Pat. No. 4,304,902. These copolymers provide enhanced aqueous viscosity, but do not provide good flow and leveling in latex paints.
U.S. Pat. No. 4,411,819 describes the preparation of polyethers which have branched chain structure and are characterized by having terminal hydrophobes. They are made by reacting a low molecular weight polyol with a mixture of ethylene oxide and at least one lower alkylene oxide having 3-4 carbon atoms. The polyethers are then end-capped with a mixture of C.sub.12 -C.sub.18 alpha-olefin oxides.
Low molecular weight (.about.9,000) hydrophobically end-capped polyether is disclosed in PCT Int. Appl. WO 92 08753. These are made by coupling low molecular weight (.about.4,500) surfactant with m-dichloromethylbenzene. Preparation of low molecular weight (.about.9,000) hydrophobe end-capped polyethers is also disclosed in U.S. Pat. No. 5,045,23. These are made by reacting C.sub.8 -C.sub.22 aliphatic alcohols with a mixture of ethylene oxide and propylene oxide and subsequently coupling the alkoxylated alcohols with a diepoxide to form a polyether (MW .about.9,000). Since these hydrophobically end-capped polyethers are of low molecular weight, they do not efficiently viscosify aqueous systems including latex paints.
U.S. Pat. No. 5,574,127 discloses associative thickeners which are water soluble polymer compositions that have a backbone of poly(acetal- or ketal-polyether) with ends that are capped with hydrophobic groups. They are prepared by copolymerizing an alpha, omega-diol, -dithiol or -diamino polyether with a gem-dihalide compound in the presence of a base to form an alpha, omega-diol, -dithiol or -diamino poly(acetal- or ketal-polyether) which in turn is reacted with hydrophobic reagents to form the final product. These associative thickeners are used in film forming coating compositions, such as latex paints.
An alternative strategy to achieve better rheological properties for various water-borne systems is to rely on the properties of associative thickeners with a comb architecture. Comb type polyurethane thickeners are described in U. S. Pat. No. 5,496,908. These polymers have moderate viscosities in aqueous solution and are useful as thickeners for alkyd- or latex-based paints. Also reported are synthesis and viscoelastic properties of comb type polyurethane thickeners (Xu, B. et al, Langmuir, 1997, 13, 6896; Xu, B. et al, Langmuir, 1997, 13, 6903).
It is known in the art that currently there is no single thickener that provides all the desired performance characteristics required in water-borne coatings. Hence, very often, attempts are made to use blends of two or more different thickeners to achieve the targeted coating rheology. While this approach works in a limited way, blending of thickeners is often cumbersome and depending on mutual interactions between the individual thickener, the stability and performance of the coatings could be in jeopardy.
Despite the continued activity in the art, there has not been produced a hydrophobically modified synthetic water soluble polymer which is hydrolytically stable over a wide pH range and at elevated temperatures and useful for viscosifying aqueous compositions in a cost-effective manner.